Manaea, 25, has been sidelined nearly three weeks because of tightness in his pitching shoulder. He went 1-2 with a 5.18 ERA in his first five starts this season before being placed on the DL on April 27.
Manaea told the San Jose Mercury News this past weekend that he feels "as good as I've felt all year" after a minor league rehab assignment. He threw four innings last Tuesday at Triple-A Nashville, allowing one earned run with five strikeouts.
To make room for Manaea on their roster, the Athletics optioned outfielder Bruce Maxwell to Nashville.